1. Field of the Invention
The present invention realtes to a differential power transmission device for tape recorders such as digital audio tape recorders and video cassette tape recorders, and more particularly to a differential transmission device wherein the rotation force of a drive motor can be more surely transmitted to a driven gear.
2. Description of the Prior Art
Referring to FIGS. 1A and 1B, there is shown a differential power transmission device generally used for tape recorders such as digital audio tape recorders(hereinafter, referred to as DAT) and video cassette tape recorders(hereinafter, referred to as VCR). As shown in the drawings, the differential power transmission device comprises a pulley gear 4 rotatably mounted to a shaft 2 fixed to a main base 1 of a recorder deck body(not shown) and adapted to receive the drive force of a drive motor(not shown) by means of a belt 3 connected between the pulley gear 4 and the shaft of the drive motor, an upper gear 6 fixedly mounted to a shaft 5 rotatably mounted to a second main base 1' of the recorder deck body, the upper gear having a large diameter gear portion 6a always engaging with the pulley gear 4 and a small diameter gear portion 6b, a lower gear 7 rotatably mounted to the shaft 5 and having the same diameter as that of the small diameter gear portion 6b of the upper gear 6, and a driven gear 9 rotatably mounted to a shaft 8 fixed to one side of a third main base 1" of the recorder deck body, the driven gear always engaging with the lower gear 7. A boss 12 is rotatably mounted to a shaft 10 fixed to the other side of the third main base 1". The boss 12 has at its lower portion a support plate 11 which is provided at its opposite sides with engaging pieces 11a and 11b. Around the boss 12, a torsion spring 13 is fitted and supported at its both ends to engaging pieces 11a and 11b, respectively. To one side of the support plate 11, namely the engaging piece 11b, a tension coil spring 14 is connected which is adapted to always urge the support plate 11 to rotate about the shaft 10 in clockwise. The differential power transmission device also comprises a connecting gear 16 rotatably mounted to a shaft 15 fixed to the protion of support plate 11 opposite to the engaging piece 11b. The connecting gear 16 is selectively engaged with both the small gear portion 6b of the upper gear 6 and the lower gear 7, so as to transmit the rotation force of the pulley gear 4 to the driven gear 9. To the rear portion of the recorder deck body, a middle plate 17 is fixedly mounted. to which an operating plate 18 is mounted to reciprocate laterally. The operating plate 18 has at its one end a foot piece 18a.
The conventional differential power transmission device having the above-mentioned construction is designed to transmit the drive force of drive motor to the driven gear 9, only when the transmission is required. Under the condition that the foot piece 18a is disposed apart from the end 13b of the torsion spring 13, as shown in FIGS. 1A and 1B, the support plate 11 is urged to rotate about the shaft 10 in clockwise, by virtue of the tension force of the tension coil spring 14. As a result, the connecting gear 16 is maintained to be disengaged from both the small gear portion 6b of the upper gear 6 and the lower gear 7, thereby preventing the drive force of drive motor from being transmitted to the driven gear 9.
As the operating plate 18 is moved in the direction indicated by the arrow A in FIG. 1A, according to an operation mode reversal from the above-mentioned condition, the operating plate 18 pushes the end 13b of the torsion spring 13 by its foot piece 18a. At this time, the shifted amount of the torsion spring 13 functions as the force rotating the support plate 11 about the shaft 10 in anti-clockwise. As a result, the connecting gear 16 is engaged with both the small gear portion 6b of the upper gear 6 and the lower gear 7, so that the rotation force of the pulley gear 4 can be transmitted to the driven gear 9, via the upper gear 6 and the lower gear 7.
In the power transmission device, however, the small gear portion 6b of the upper gear 6 and the lower gear 7 which are simultaneously engaged with the connecting gear 16 may not be aligned with each other, as shown in FIG. 3, thereby interfering with their smooth engagement with the connecting gear 16. As a result, the upper gear 6 may be idly rotated.